A data processing system typically comprises a central processing unit (CPU) and a storage subsystem linked through an appropriate interface and control system. The subsystem includes a plurality of storage units such as high capacity direct access storage devices (DASDs). An economical alternative to high-end DASD is a subsystem comprising redundant arrays of inexpensive disks (RAID). A number of RAID architectures are described in a foundational article entitled "A Case for Redundant Arrays of Inexpensive Disks (RAID)", by D. A. Patterson et al., Report No. UCB/CSD 87/391, December 1987, incorporated herein by reference.
A number of early storage subsystems provided redundant secondary devices for duplicating data as a back up to primary devices in the event of drive failure. Such redundancy improves system reliability, but substantially decreases the data capacity of the subsystem because substantial space is required for duplicated data.
In a more efficient redundancy method, data is "striped" or distributed across the plural devices of a storage array rather than being written to a single storage device. For example, a storage system may include N+P devices. Each device further includes a plurality of predefined data regions that may be the size of a disk block, multiple disk blocks, a track, a cylinder, etc. An incoming data stream comprises N logically consecutive data blocks which are distributed to corresponding addresses of N array devices. The blocks are the same size as the predefined data regions and are distributed such that no two blocks of data from the same transfer unit reside on the same device. Parity information is generated for the N blocks and is stored on a separate device P. A group of data blocks and corresponding parity are called a parity group and are represented as (N+P). The size of the parity group is determined by the number of devices, N+P, in the subsystem.
In the event of a single device failure, no more than one data block of each parity group will be lost, due to the manner in which stored data and parity are distributed. The lost data can be reconstructed from the remaining data and parity blocks residing on other devices. The combination of striping and parity thus assures that a single device failure will not result in unrecoverable data loss. Data distribution over a plurality of failure independent DASDs is disclosed in commonly assigned U.S. Pat. No. 4,092,732 to Ouchi. Benefits of data striping are discussed further in a paper entitled "Disk Striping" by K. Salem and H. Garcia-Molina published in the IEEE 1986 Int'l. Conf. on Data Engineering, pp. 336-342.